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temperature materials used in gas turbine engines.
For this class of work, high purity argon shielding gas
is fed to both sides of the weld and the welding torch
nozzle is fitted with a gas lens to ensure maximum
efficiency for shielding gas coverage. A consumable
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Fig. 22-6 Wide chord fan blade
construction.
Fig. 22-7 Typical tungsten inert gas
welding details.
four per cent thoriated tungsten electrode, together
with a suitable non-contact method o! arc starting is
used and the weld current is reduced in a controlled
manner at the end of each weld to prevent the
formation of finishing cracks. All welds are visually
and penetrant inspected and in addition, welds
associated with rotating parts i.e., compressor and/or
turbine are radiologically examined to Quality
Acceptance Standards. During welding operations
and to aid in the control of distortion and shrinkage
the use of an expanding fixture is recommended
and, whenever possible, mechanised welding
employed together with the pulsed arc technique is
preferred. A typical T.I.G. welding operation is
illustrated in fig. 22-8.
Electron beam welding (E.B.W.)
29. This system, which can use either low or high
voltage, uses a high power density beam of
electrons to join a wide range of different materials
and of varying thickness. The welding machine ref.
fig. 22-9, comprises an electron gun, optical viewing
system, work chamber and handling equipment,
vacuum pumping system, high or low voltage power
supply and operating controls. Many major rotating
assemblies for gas turbine engines are manufactured
as single items in steel, titanium and nickel
alloys and joined together i.e., intermediate and high
pressure compressor drums. This technique allows
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Fig. 22-8 Tungsten inert gas welding.
Fig. 22-9 Electron beam welding.
design flexibility in that distortion and shrinkage are
reduced and dissimilar materials, to serve quite
different functions, can be homogeneously joined
together. For example, the H.P. turbine stub shafts
requiring a stable bearing steel welded to a material
which can expand with the mating turbine disc.
Automation has been enhanced by the application of
computer numerical control (C.N.C.) to the work
handling and manipulation. Seam tracking to ensure
that the joint is accurately followed and close loop
under bead control to guarantee the full depth of
material thickness is welded. Focus of the beam is
controlled by digital voltmeters. See fig. 22-10 for
weld examples.
ELECTRO-CHEMICAL MACHINING (E.C.M.)
30. This type of machining employs both electrical
and chemical effects in the removal of metal.
Chemical forming, electro-chemical drilling and electrolytic
grinding are techniques of electro-chemical
machining employed in the production of gas turbine
engine components.
31. The principle of the process is that when a
current flows between the electrodes immersed in a
solution of salts, chemical reactions occur in which
metallic ions are transported from one electrode to
another (fig. 22-11). Faraday’s law of electrolysis
explains that the amount of chemical reaction
produced by a current is proportional to the quantity
of electricity passed.
32. In chemical forming, (fig. 22-11), the tool
electrode (the cathode) and the workpiece (the
anode) are connected into a direct current circuit.
Electrolytic solution passes, under pressure, through
the tool electrode and metal is removed from the
work gap by electrolytic action. A hydraulic ram
advances the tool electrodes into the workpiece to
form the desired passage.
33. Electrolytic grinding employs a conductive
wheel impregnated with abrasive particles. The
wheel is rotated close to the surface of the
workpiece, in such a way that the actual metal
removal is achieved by electro-chemical means. The
by-products, which would inhibit the process, are
removed by the sharp particles embodied in the
wheel.
34. Stem drilling and capillary drilling techniques
are used principally in the drilling of small holes,
usually cooling holes, such as required when
producing turbine blades.
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Fig. 22-10 Examples of T.I.G. and E.B. welds.
Stem drilling
35. This process consists of tubes (cathode)
produced from titanium and suitably insulated to
ensure a reaction at the tip. A twenty per cent
solution of nitric acid is fed under pressure onto the
blade producing holes generally in the region of
0.026 in. diameter. The process is more speedy in
　
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